In oil drilling, it is often necessary to remote actuate tools located in a wellbore.
According to the prior art, an annular piston with two faces and a throttle device comprising a needle choke with a variable flow section are used. One face of this piston is subjected to the pressure forces prevailing on one side of the throttle device and the other face is subjected to the pressure forces on the other side of the throttle device.
Generally, the choke is borne by the piston and the needle is set in relation to a pipe containing the assembly and in which the piston may move so as to achieve the desired actuation. The piston includes return means which maintain it in an idle position corresponding to a relatively wide passageway of the throttle device leading to a low pressure drop for the working flow rates.
When the equipment is to be actuated, the rate of flow is increased, which increases the pressure drop on either side of the throttle device, and the piston therefore tends to move by acting against the return means. During this motion, the choke enters the throttle device more and more deeply, hence a greater increase in the pressure drop providing the power for actuating the equipment.
The prior art may be illustrated by Patent FR-2,575,793.
Such a device lacks precision as far as the threshold flow rate initiating the actuation is concerned. In fact, the assembly consisting of the piston and the return spring, which must react to or transmit high powers, cannot be precisely sensitive to a given threshold flow rate, for example because of frictional stresses.
Moreover, this device works through a flow rate increase with respect to the working flow rates. Now, drilling conditions may forbid such a flow rate increase. In fact, the increase due to pressure drops downstream from the device may lead to fracturations in the formation or destabilize the well walls, which might challenge the safety of the operation. On the other hand, a power increase with respect to the power used for drilling is often impossible because the pumping equipment is already prompted at full power for the drilling operation itself.
Patent FR-2,641,320 solves the problem linked to the precision of the threshold flow rate by using a choke or a needle borne by the piston, but mobile with respect to the piston.
This choke or this needle, of a smaller size with respect to the piston and provided with suitable return means, is precisely sensitive to a flow rate threshold, but its main drawback is that the actuation is still initiated by a flow rate increase with respect to the working flow rates.
Patent FR-2,670,824 describes a device allowing these two problems to be solved by using a needle-choke or an equivalent system. This device allows notably the actuation to be initiated from a flow rate threshold less than or equal to the working flow rates, while providing an appreciable activating force such as that necessary for the actuation.
Document FR-2,670,824, filed by the applicant, discloses an actuating device in which a system for sealing partly the fluid passageway is adjusted according to two positions: an actuating position and a position called a drilling position, where no actuation is possible. The adjustment is either remote controlled from the surface or it occupies successively the two positions through the use of an appropriate operational sequence. The drawback of this device is that it requires a complex remote control system and, in the other case, the procedure does not give reliable information on the real position of the seal system.